Cooling water treatment system
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of our copending application Ser. No. 07/420,115, filed Oct. 10, 1989, entitled &#34;COOLING WATER TREATMENT SYSTEM&#34;. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to improved formulations for cooling water, and to the in-service treatment and maintenance of systems using relatively large quantities of cooling water, and more specifically to such a formulation which may be utilized to both restore system performance through removal of scale on an in-service basis, while at the same time being designed to maintain the system in proper condition through subsequent prevention or substantial elimination of build-up or redeposition of additional scale. The present invention is an improvement over that disclosed and claimed in the prior U.S. Patent of Bruce D. Busch, No. 4,279,768, granted July 21, 1981, entitled &#34;SERVICE DESCALER FOR AQUEOUS SYSTEMS&#34;, and assigned to the same assignee as the present invention. 
     In the operation of systems utilizing cooling water, quantities of water as required are introduced and utilized at a rate dependent upon the service requirements of the installation. As introduced, make-up water universally contains a quantity of impurities and/or contaminants which, unless treated, contribute to a constant build-up of scale on the heat transfer surfaces or working surfaces of the system. Normal operation of these systems results in the consumption of reasonably large quantities of water, and as water is lost from the system, normally through evaporation, levels of concentration of these contaminants and/or impurities increase. These contaminants and/or impurities are normally those same elements or compounds which commonly contribute to water hardness, specifically certain calcium salts along with certain quantities of iron oxides. As has been recognized in the past, scale build-up on heat transfer surfaces in the aforementioned equipment causes ongoing expense to the owner as well as possible exposure to dangerous conditions for those persons located near such equipment. Scale build-up increases thermal impedance and thus decreases the rate of heat transfer through the surfaces. This contributes to an overall loss of efficiency of the system as well as increased energy consumption and costs. Scaled heat transfer surfaces ar susceptible to corrosion beneath the scaled area since corrosion control agents are unable to effectively contact the metal surfaces. Furthermore, scale acts as an insulative barrier, resulting in inefficient heat transfer. This condition leads to both increased energy costs and decreased system efficiency. System component failures can occur due to the higher than normal temperatures being required at the heat transfer surfaces in order to maintain normal operation and/or operation requiring extraordinary performance. Thus, clean scale-free heat transfer surfaces represent a desirable condition in cooling water systems. 
     In the prior U.S. Pat. No. 4,279,768 mentioned above, an in-service descaler is disclosed containing a number of the components present in the present formulation. The present invention constitutes an optimization of those disclosed in U.S. Pat. No. 4,279,768, and furthermore incorporates other components which contribute to the effectiveness of the present formulation for cooling water treatment while the system remains on-line. The primary components of most water impurity based scales in the equipment mentioned above are calcium salts (e.g. calcium carbonate, calcium orthophosphate, calcium sulfate and calcium bicarbonate) as well as iron oxides. These compounds are present in most domestic and commercial water supplies, being normally found in surface and in subterranean water sources. 
     The present material is particularly adapted for use in reducing scale, as well as inhibiting corrosion in open recirculating cooling water systems. Furthermore, the formulation of the present invention is free of chromate, and the formulations eliminate the requirement for the addition of quantities of sulfuric acid. Corrosion control is provided with this system, for both ferrous and non-ferrous (cuprous) metals. 
     The presently commercially available and widely utilized cooling water scale prevention includes formulations utilizing chelants, polymeric conditioners and certain gluconates as sequestering agents. 
     The present system utilizes chelants such as tetrasodium ethylene diamine tetraacetic acid (EDTA) and trisodium nitrilotriacetate (NTA) to stoichiometrically sequester calcium and iron ions. Stoichiometric treatments require significant concentrations to prevent scale formation, particularly when the water source contains substantial quantities of calcium and/or iron ions. Gluconates and polyelectrolytes are normally threshold treatments. Threshold treatments are generally effective in controlling large numbers of ions per unit of treatment, and the economy of using such threshold treatments is readily apparent. Specific modified formulations in use today are disclosed in U.S. Pat. No. 4,279,768. 
     In addition to the formulations of U.S. Pat. No. 4,279,768, scale removal has been accomplished by descaling the thermal energy transfer surfaces of cooling towers and the like with acid treatments. Organic acids such as citric and sulfamic or mineral acids such as sulfuric and/or hydrochloric are frequently used to remove scale of the type mentioned above. However, in acid descaling, the system must normally be shut down, drained, acid cleaned, rinsed, drained for a second time and retreated. Acid descaling thus requires either waiting for a convenient time to shut down operations o possibly interrupting production schedules on an emergency basis to achieve cleaning. Either alternative is known to be costly, since draining of the system is often required, and an added cost is the replenishing of chemical treatment once the system is clean. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide formulations which are capable of in-service use for scale removal as well as for prevention of deposition of additional scale. The present formulations find utility in a wide variety of water cooling applications, with most of these applications being characterized as &#34;cooling tower&#34; applications. Thus, the term &#34;cooling tower&#34; as employed herein is utilized in a comprehensive sens and is intended to include those systems in which water is utilized as a fluid or liquid coolant over heat transfer surfaces, primarily those heat transfer surfaces employed in evaporative/condensers in large scale air-conditioning systems. The descaling and surface maintenance procedures may be accomplished by the formulation of the present invention while the system remains in operation. No system shut-down or draining operations are necessary when using the formulation of the present invention. The formulation dissolves and chelates calcium scales and iron oxides from scale which may have already been formed on the heat transfer surfaces of the system while the system remains in operation. In other words, these dissolved calcium scales and iron oxides are maintained in solution while normal cooling operations are permitted to continue. The maintenance operation accordingly is achieved in an economic fashion. 
     The formulations of the present invention are accordingly designed to perform the following: 
     1. Remove scale deposits from cooling tower systems while the systems remain on-line; 
     2. Offer an increased capability to prevent redeposition of the removed scale; and 
     3. Permit cooling tower systems to be run at greater cycles of concentration without encountering redeposition of scale solids. This combination of features permits on-going operation of systems without requiring shut-down, while continuing to maintain system performance and thus results in fuel/energy, water costs and chemical cost savings, thus increasing fuel/energy savings. 
     The present invention comprises a synergistic blend of a number of functional components, including scale-preventing, modifying and conditioning components, a chelant, and ferrous and non-ferrous (copper) metal corrosion inhibitors. 
     The scale-preventing, modifying and conditioning ingredients include one or more phosphonates selected from the group consisting of 1-hydroxyethane-1,1-diphosphonic acid (HEDP), aminotris methylene phosphonic acid (AMP), and 2-phosphonobutane-1,2,4tricarboxylic acid (PBTC). One or more scale-modifying polymers are utilized, with these polymers being selected from the group consisting of phosphinocarboxylic acid derivatives, polymaleic acid derivatives, polyacrylic acid derivatives, and sulfonated-styrene maleic acid compounds. These polymers are commercially available. In addition, sodium gluconate is utilized primarily for its ferrous metal corrosion inhibiting properties, as well as its ability to remove scale when present in the formulation of the present invention. Sodium molybdate is optionally but preferably present, particularly as a secondary ferrous metal corrosion inhibitor. Sodium benzotriazole or sodium tolytriazole is optionally, but preferably, utilized as a copper metal corrosion inhibitor. 
     The chelant or chelants utilized are preferably selected from the group consisting of tetrasodium salt of ethylene diamine tetraacetic acid (tetrasodium EDTA) or trisodium nitrilotriacetate (trisodium NTA). The EDTA and NTA chelants are selected because of their high efficiency in the formulation, particularly in combination with the other components, as well as their commercial availability and low cost. 
     Therefore, it is a primary object of the present invention to provide an improved in-service descaler formulation for use in cooling water, and which has been found highly effective for in-service treatment of cooling tower installations and systems. 
     It is a further object of the present invention to provide an improved formulation for the removal of scale from heat transfer surfaces of cooling towers, and for the ongoing maintenance of these surfaces while the systems are maintained in continuous operation. 
     It is yet a further object of the present invention to provide an improved formulation for both scale removal and continued treatment of cooling tower surfaces while the system remains on-line. 
     It is yet further object of the present invention to provide an in-service descaler and cooling tower maintenance product wherein the formulation provides for protection of both ferrous and non-ferrous metal surfaces from attack and/or corrosion. 
     Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification and appended claims. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order to best disc lose the properties of the preferred embodiment, the following specific formulation is provided: 
     
         ______________________________________                                    
Ingredient             Percent by Weight                                  
______________________________________                                    
2-phosphonobutane-1,2,4-                                                  
                       10.0%                                              
tricarboxylic acid                                                        
(PBTC phosphonate)                                                        
(50% aqueous solution)                                                    
1-hydroxyethane-1,1-   2.77%                                              
diphosphonic acid                                                         
(HEDP phosphonate)                                                        
(60% aqueous solution)                                                    
polymaleic acid (50% aqueous solution)                                    
                       5.00%                                              
having a molecular weight of between                                      
500 and 1000 (available commercially                                      
under the trade name &#34;Belclene 200&#34;                                       
form Ciba-Geigy Corporation of                                            
Ardsley, NY)                                                              
acrylate copolymer (40% aqueous solution)                                 
                       6.25%                                              
having a molecular weight of                                              
approximately 4500 (available commercially                                
under the trade name &#34;Acumer 2000&#34; from                                   
Rohm and Haas Company)                                                    
polyacrylate polymer   2.00%                                              
(50% aqueous solution) having a molecular                                 
weight of approximately 5100 (available                                   
commercially under the trade name                                         
&#34;Good-Rite K-732&#34; from B. F. Goodrich                                     
Company)                                                                  
sodium gluconate       1.00%                                              
tetrasodium salt of ethylene                                              
                       10.0%                                              
diamine tetraacetic acid                                                  
(40% aqueous solution) (Na.sub.4 EDTA)                                    
sodium molybdate       2.00%                                              
sodium benzotriazole   0.33%                                              
water                  balance.                                           
______________________________________                                    
 
    
     The above formulation has been found to be effective at scale removal and prevention in cooling tower applications when present in the cooling water at concentration ranges between about 20 and 100 ppm. 
     In order to provide a selection basis for formulations capable of achieving both an initial descaling operation as well as a cooling tower maintenance material, a number of comparative tests were undertaken. These tests are outlined hereinbelow: 
     TEST PROCEDURE 
     Testing the scale removing capacity of the formulation of the present invention was undertaken by assembling a laboratory test rig. The test rig held constant temperature, flow rate and water volume. The test rig monitored and recorded conductivity values of the test solutions as testing proceeded. A total of six gallons of distilled water were added to the test rig and heated to 100° F. Next, 250 ml of product to be tested were added to the rig solution. Thereafter, 5 grams of calcium carbonate were added to the rig solution and conductivity change was monitored and recorded for one hour. An additional 5 grams of calcium carbonate were added and, again, conductivity changes were monitored for one hour. An increase in conductivity represents the dissolving of calcium carbonate and is therefore representative of the deposit removal capacity of the treatment. 
     TEST A 
     An in-service descaler of the type disclosed and claimed in U.S. Pat. No. 4,279,768 was prepared with the following formulation: 
     
         ______________________________________                                    
                         Percent                                          
Component                by Weight                                        
______________________________________                                    
Chelant (tetrasodium ethylene                                             
                         10.0                                             
diamine tetraacetic acid),                                                
40% by weight aqueous solution,                                           
commercially available                                                    
Antiprecipitant/dispersant (an                                            
                         0.25                                             
anionic organic polyelectrolyte                                           
(polyacrylamide) having a molecular                                       
weight of about 8000) available                                           
commercially under the trade name                                         
&#34;Liquid Cyanamer P-35&#34; from                                               
American Cyanamid Company, Industrial                                     
Chemicals Division of Wayne, NJ)                                          
Antiprecipitant (the anionic                                              
                         0.25                                             
organic polyelectrolyte                                                   
(polyacrylamide) having a molecular                                       
weight of about 1000) available                                           
commercially under the trade name                                         
&#34;Liquid Cyanamer P-70&#34; from                                               
American Cyanamid Company, Industrial                                     
Chemicals Division of Wayne, NJ)                                          
Non-ferrous metal corrosion inhibitor                                     
                         0.50                                             
(benzotriazole) (available commercially                                   
under the trade name &#34;Cobratec 99&#34; from                                   
Sherwin Williams Company of New York, NY)                                 
Ferrous metal corrosion inhibitor                                         
                         1.0                                              
(sodium gluconate) available                                              
commercially                                                              
water                    balance.                                         
______________________________________                                    
 
    
     With this material in the test rig described above, an increase of 90 micromhos was observed. This increase in conductivity represents scale removal capacity as previously disclosed in the prior patent mentioned. 
     TEST B 
     A cooling water treatment was prepared with the following formulation: 
     
         ______________________________________                                    
Ingredient           Percent by Weight                                    
______________________________________                                    
2-phosphonobutane-1,2,4-                                                  
                     10%                                                  
tricarboxylic acid (PBTC                                                  
phosphonate) (50% solution)                                               
1-hydroxyethane-1,1-diphosphonic                                          
                     3%                                                   
acid (HEDP phosphonate)                                                   
(60% solution)                                                            
polymaleic acid, sodium salt                                              
                     5%                                                   
(50% solution)                                                            
polyacrylate copolymer (molecular                                         
                     6%                                                   
weight of 10,000) (50% solution)                                          
sodium molybdate     2%                                                   
sodium benzotriazole   0.5%                                               
water                balance.                                             
______________________________________                                    
 
    
     With the above formulation in the test rig, no increase in conductivity was noted. The cooling water treatment was therefore not shown to possess scale removing capabilities. 
     TEST C 
     A combination of in-service descaler as in Test A and cooling water treatment as in Test B was prepared by having the following formulation: 
     
         ______________________________________                                    
Ingredient           Percent by Weight                                    
______________________________________                                    
2-phosphonobutane-1,2,4-                                                  
                     10%                                                  
tricarboxylic acid (PBTC                                                  
phosphonate) (50% solution)                                               
1-hydroxyethane-1,1-diphosphonic                                          
                     3%                                                   
acid (HEDP phosphonate)                                                   
(60% solution)                                                            
polymaleic acid, sodium salt                                              
                     5%                                                   
(50% solution)                                                            
polyacrylate copolymer (molecular                                         
                     6%                                                   
weight of about 10,000)                                                   
(50% solution)                                                            
polyacrylate (50% solution)                                               
                     2%                                                   
(molecular weight of between                                              
2000-6000)                                                                
sodium gluconate     1%                                                   
tetrasodium salt of ethylene                                              
                     10%                                                  
diamine tetraacetic acid                                                  
(40% solution)                                                            
sodium molybdate     2%                                                   
sodium benzotriazole   0.5%                                               
water                balance.                                             
______________________________________                                    
 
    
     With the above formulation in the test rig, an increase in conductivity of 280 micromhos was noted. This conductivity increase represents a greater than three-fold increase in scale removal capacity over the Test A and Test B formulations. Since none of the ingredients present in the Test C formulation were included at a greater percentage than in the Test A and Test B formulations, the increase in scale removing performance may be be attributed to a synergism between the ingredients in Test formulation A and Test formulation B as disclosed in Test formulation C. 
     In order to employ the formulation of the present invention in an actual working solution, the formulation may be introduced into the cooling tower water by any suitable means, preferably an injector. Typically, injectors are calibrated so as to deliver a predetermined and fixed proportion of the injected material to the make-up water. 
     In actual operation, system conditions for cooling water treatment will normally be in accordance with the following schedule: 
     
         ______________________________________                                    
SCHEDULE                                                                  
______________________________________                                    
Total Hardness    1000   ppm maximum                                      
Silica            175    ppm maximum                                      
Sulfate           1000   ppm maximum                                      
&#34;M&#34; alkalinity    750    ppm maximum                                      
pH                9.0    maximum                                          
Langlier Index    3.0    maximum                                          
______________________________________